In a typical turbine engine, air is compressed, then mixed with fuel, and the resulting mixture is ignited in a combustor, so that the expanding gases of combustion can rapidly move across and thus rotate the turbine blades. The fuel can be liquid (e.g., Diesel Fuel #2) or gaseous (e.g., methane) or both, and the turbine can be an axial flow or a radial in-flow type. Such turbine engine can be used for industrial power or moving an airplane or ground vehicle. Variable or fixed turbine vanes direct the expanded gases from the combustor to the rotatable turbine blades.
Air polluting emissions are an undesirable bi-product of turbine engines. The primary air pollution emissions produced by turbines burning conventional hydrocarbon fuels are oxides of nitrogen (NO.sub.x), carbon monoxide (CO) and unburned hydrocarbons. It is well known that oxidation of molecular nitrogen in air-breathing engines is dependent upon the flame temperature in the reaction zone. The rate of chemical reactions forming oxides of nitrogen is an exponential function of temperature. Consequently, if the flame temperature is controlled to a low level, thermal NO.sub.x production will be reduced.
A typical and preferred method of controlling the temperature of the reaction zone of a turbine combustor below the level at which thermal NO.sub.x is formed consists of pre-mixing the fuel and air to a lean mixture prior to combustion. The mass of the excess air present in the reaction zone of a lean, pre-mixed combustor absorbs heat and reduces the temperature rise of the products of combustion to a level where NO.sub.x production is substantially reduced. However, the fuel/air mixture strength should be somewhat higher than the lean flammability limit in order to prevent or eliminate combustion oscillations. It is generally known that lean, pre-mixed combustors tend to be less stable than more conventional diffusion flame combustors and do not provide adequate turn down for operation over the entire load range of the turbine. Stability for operation over all load conditions required for turbine operations, with minimum emissions of air pollutants in the turbine exhaust, is an ongoing challenge in the industry.
For liquid fuel turbine engines, another challenge is that it is desirable to pre-vaporize the fuel prior to entry into the combustion chamber. Pre-vaporizing the liquid fuel maximizes the combustion efficiency of the engine and minimizes pollution and stability problems. However, it is believed that in even the most efficient systems, full pre-vaporization of the fuel has not been achieved, that is, the fuel is not completely pre-mixed at the molecular level with the air prior to combustion. Consequently, flame temperature and NO.sub.x formation rates are higher than what is believed achievable in fully pre-mixed, pre-vaporized systems. Steam and/or water are many times injected into the. combustor primary zone to reduce and control formation of the oxides of nitrogen. However, the additional requirement of a steam and/or water injection system greatly increases the capital operating and maintenance costs of the turbine.
Another method of NO.sub.x control is with the use of catalytic combustors. This technique also raises capital, operating and maintenance costs issues with the turbine. There are also technological issues, such as material and structural integrity of the catalyst under high temperature and thermal cycling conditions, which must be resolved. It is also believed that the use of catalytic combustion has not been successfully demonstrated for oil fired combustion turbines.
As such, it is believed that there is a demand in the industry for an improved fuel injection apparatus for a turbine combustion system, where the system has clean and stable operation, and which does not require secondary control of NO.sub.x formation.